Lewis structure of I3-

Te Jung Yang 4K
Posts: 63
Joined: Fri Sep 28, 2018 12:18 am

Lewis structure of I3-

I don't understand how to make the I3- lewis structure.
The correct lewis structure puts the formal charge -1 on the center I, while I put the formal charge -1 on the side I (via a double bond, creating a resonance structure).
I did this because I figured that the formal charge would be the best further away from electron densities (in this case, the other Is), minimizing e- repulsion. Furthermore, an electron pair is more stable in between nuclei.
Is there any flaws in my reasoning? Would someone explain why putting the formal charge of -1 on the center I is correct?

CateJensen3K
Posts: 32
Joined: Fri Sep 28, 2018 12:26 am

Re: Lewis structure of I3-

I believe the reason why is because when you put the formal charge on the central atom, the outer Is have a formal charge of 0 which makes entire ion more stable. When drawing lewis structures, I usually start by counting all of the valence electrons including the -1 charge. So for I, the valence electron number is 7 and since there are three Is I would add 7+7+7+1= 22.
Then I draw the lewis structure with single bonds between the Is in order to have a sort of skeleton to work with (I-I-I), this gives me a total of 4 electrons. After that you need to fill in the remaining amount of electrons. So place 3 pairs of electrons on each I in order to get to 22. The reason that you wouldn't want to use a double bond is because you want the formal charge to fall on the central atom and you want the outer atoms to have a formal charge of 0. If this was I3, the central atom would only have two pairs of electrons.

I hope this helps!